The Limits of Genetic Testing

Are diseases genetic? That's the simplified and distorted mantra we hear every day in the media -- that scientists have just discovered the gene causing this or that disease.

The truth is that genes only very rarely cause diseases. An illuminating new study in the journal Science Translational Medicine helps clarify what geneticists have been trying to explain to us for years: genes influence, but they don't determine.

The just-published study examines how often identical twins get the same diseases. Reviewing records of 53,666 identical twins in the United States, Sweden, Finland, Denmark, and Norway, researchers tabulated how well genes predict the chance of getting a disease. The answer is that they really can't. Predictions based on genes turned out to be very close to useless. As Gina Kolata summed up in TheNew York Times: "While sequencing the entire DNA of individuals is proving fantastically useful in understanding diseases and finding new treatments, it is not a method that will, for the most part, predict a person's medical future."

Both the study and Times reporting are refreshing. For years, twin studies have been used to convince the public how strongly our traits are based in genetics. Many of us have argued that such "heritability" studies were gross distortions of the genetic reality. Now perhaps twin studies can be used to show the actual relationship between genes and development.

Here's the reality of how genes really work: Genes are not blueprints with elaborate preset instructions for eye color, thumb size, mathematical quickness, musical sensitivity, breast cancer, etc. Instead, they are more like volume knobs and switches. Think of a giant control board inside every cell of your body. Many of those knobs and switches can be turned up/down/on/off at any time -- by another gene or by a minuscule environmental input. This flipping and turning -- called "gene expression" -- takes place constantly. It begins the moment a child is conceived and doesn't stop until she takes her last breath. Rather than giving us hardwired instructions on how a trait must be expressed, this process of gene-environment interaction drives a unique developmental path for every unique individual.

Gene expression means that you could have the exact same gene as someone else -- as identical twins do -- but have no way of knowing what the actual effect of that gene is going to be. It will depend on various developmental life circumstances. We could clone Einstein and we really don't know if he's going to turn out to be an Einstein.

Remember gene expression the next time someone mentions an "innate musical talent," or a "natural-born swimmer," or "the math gene." As a general rule, traits and diseases are developmental, not gene-determined.

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David Shenk is a writer on genetics, talent and intelligence. He is the author of Data Smog, The Forgetting, and most recently, The Genius In All of Us.